At the time of allotment of shares on the exercise date, the difference between fair market value of the shares as on exercise date and the amount that employee have paid for the exercise or subscripttion to the shares is calculated and taxed accordingly. This taxable value is called Perquisite value. This difference calculated is eligible for TDS deduction by the company and forms part of salary of the employee which is shown in Form 16 and Form 12BA of the employee.
When employee opts to sell the shares previously allotted under the ESOP, profits made by him are taxed as capital gains earned during the year. These Capital Gains are calculated by subtracting the fair market value as on the exercise date from the sales consideration of such shares.